The present invention relates to a resin composition for an aqueous paint, which contains substantially no volatile organic compound (hereinafter referred to as xe2x80x9cVOCxe2x80x9d) and has excellent freezing-thawing stability and low temperature film-formability and forms a film excellent in anti-blocking property and water resistance.
Recently, environmental regulations became stricter with regard to reduction of toxicity and a VOC, and also from a viewpoint of saving resources, it is demanded to use water in place of an organic solvent as a solvent for a paint in the field of paint business. Its typical paint is an aqueous emulsion paint, but since the aqueous emulsion paint employs water as a solvent, there are problems that its dispersion state is changed by freezing-thawing cycle during storage and freezing-thawing stability is poor due to viscosity rise or coalescing. Therefore, it is heretofore general to employ an organic solvent including ethylene glycol as a freeze-proofing agent, thereby improving freezing-thawing stability.
Also, an aqueous emulsion resin has an inherent minimum film-forming temperature (hereinafter referred to as xe2x80x9cMFTxe2x80x9d), and when a surface to be coated has a temperature of lower than MFT, it was necessary to blend an organic solvent as a coalescing agent for film-formation. Thus, the aqueous emulsion paint contains a considerable amount of a VOC, and when drying of a coated film is insufficient, there are problems of adversely affecting water resistance and anti-blocking property by the presence of the remaining VOC.
Therefore, it has been studied to reduce VOC as much as possible from an aqueous emulsion paint in order to solve environmental problems, to save resources and to prevent orders. For example, there has been developed a paint having a VOC reduced by using a low MFT aqueous emulsion resin, but the paint using a low MFT aqueous emulsion resin is poor in water resistance and anti-blocking property and provides a problem that a coated film strength is poor. Also, an aqueous emulsion paint having a low VOC amount, which is improved in freezing-thawing stability, is disclosed, for example, in JP-A-8-302238.
However, when this paint is stored in a frozen state for a long term, the freezing-thawing stability becomes poor and a coated film obtained provides problems that anti-blocking property and water resistance become poor.
The present invention has been made by taking these prior arts into consideration. Thus, an object of the present invention is to provide a resin composition for an aqueous paint for forming a coating film having excellent anti-blocking property and also having excellent freezing-thawing stability and low temperature film-formability, without using a VOC as a coalescing agent and an anti-freezing agent which are resources of providing environmental pollution and odor, or with only a very small amount of these additives.
In order to solve the above problems, the present inventors have intensively studied, and have discovered that the above problems are surely solved by providing the following constitution. The present invention is based on this discovery.
That is, the present invention relates to a resin composition for an aqueous paint, containing different phase structure emulsion particles obtained by multi-stage emulsion polymerization as a binder, wherein the different phase structure emulsion particles have an outermost phase formed by an emulsion polymer of an ethylenic unsaturated monomer having a glass transition temperature of from xe2x88x9250xc2x0 C. to 10xc2x0 C., and satisfy the following conditions (1) to (3):
(1) the emulsion polymer forming the outermost phase contains from 1 to 20 mass % of an ethylenic unsaturated monomer having at least one of a polyethylene glycol chain and a polypropylene glycol chain,
(2) at least one phase present in the inside from the outermost phase of the different phase structure emulsion particles comprises an emulsion polymer of an ethylenic unsaturated monomer having a glass transition temperature of from 30xc2x0 C. to 110xc2x0 C., and
(3) the binder has a minimum film-forming temperature of at most 10xc2x0 C.
Hereinafter, the present invention is described in more details.
A resin composition for an aqueous paint of the present invention employs different phase structure emulsion particles prepared by multi-stage emulsion polymerization of an ethylenic unsaturated monomer in water as a binder.
The multi-stage emulsion polymerization is carried out by at least 2 stages, usually from 2 to 5 stages of emulsion polymerization of an ethylenic unsaturated monomer in water in accordance with a well known emulsion polymerization process, and the emulsion polymer of an ethylenic unsaturated monomer thus formed constitutes different phase structure emulsion particles comprising the outermost phase and at least one phase of inner phases.
A typical example of the multi-stage emulsion polymerization process includes a process of a plurality of steps of repeating emulsion polymerization of an ethylenic unsaturated monomer at an elevated temperature of from 60 to 90xc2x0 C. in the presence of an emulsifying agent and a polymerization initiator, and further optionally a chain transfer agent and an emulsion stabilizer.
Examples of the emulsifier include an aliphatic acid salt such as sodium lauryl sulfate, a higher alcohol sulfuric acid ester salt, an alkylbenzene sulfonate such as sodium dodecylbenzene sulfonate, a polyoxyethylenealkylether sulfuric acid salt, ammonium polyoxynonylphenylether sulfonate, polyoxyethylene polyoxypropylene glycol ether sulfuric acid salt, a reactive emulsifier such as a monomer having a sulfonic acid group or a sulfuric ester group, and other anionic surfactants; a polyoxyethylene alkylether, polyoxynonylphenylether, a sorbitan aliphatic acid ester, a polyoxyethylene aliphatic acid ester, a polyoxyethylene-polyoxypropylene block copolymer, a reactive nonionic surfactant and other nonionic surfactants; an alkylamine salt, a quaternary ammonium salt and other cationic surfactants; (modified)polyvinyl alcohol, and the like.
Examples of the polymerization initiator include those generally used for radical polymerization, and among them, a water-soluble material is preferable, examples of which include persulfates such as potassium persulfate and ammonium persulfate, azo type compounds such as 2,2xe2x80x2-azobis(2-amidinopropane)hydrochloride, 4,4xe2x80x2-azobis-cyanovaleric acid, 2,2xe2x80x2-azobis(2-methylbutaneamidoxime)dihydrochloride tetrahydrate and the like, aqueous hydrogen peroxide, t-butylhydroperoxide and other peroxides. Further, a redox type compound including a combination of ferrous sulfate and a reducing agent such as L-ascorbic acid and sodium thiosulfate are also usable.
Examples of the chain transfer agent include long chain alkylmercaptans such as N-dodecylmercaptan, aromatic mercaptans, hydrogenated hydrocarbons and the like.
Examples of the emulsion stabilizer include polyvinyl alcohol, hydroxylethylcellulose, polyvinylpyrrolidone and the like.
The emulsion polymerization is carried out by a one time-charging method of charging a monomer one time, a monomer-dropping method of continuously dropping a monomer, a preemulsion method of previously mixing and emulsifying a monomer, water and an emulsifier and dropping the emulsified product, or a combination of these methods.
According to the present invention, in the preparation of different phase structure emulsion particles as a binder in accordance with the above-mentioned methods, as an ethylenic unsaturated monomer which is added at the final stage of multi-stage emulsion polymerization and forms the outermost phase, it is necessary to use an ethylenic unsaturated monomer containing an ethylenic unsaturated monomer having at least one of a polyethylene glycol chain and a polypropylene glycol chain in an amount of from 1 to 20 mass %, preferably from 5 to 15 mass %, of the total ethylenic unsaturated monomers in such a manner as to provide the outermost phase comprising an emulsion polymer having a glass transition temperature (hereinafter referred to as xe2x80x9cTgxe2x80x9d) of from xe2x88x9250xc2x0 C. to 10xc2x0 C., preferably from xe2x88x9230xc2x0 C. to 0xc2x0 C. Also, it is necessary to use such an ethylenic unsaturated monomer as to provide at least one phase of inner phases, comprising an emulsion polymer having a Tg of from 30xc2x0 C. to 110xc2x0 C., preferably from 34xc2x0 C. to 90xc2x0 C.
Also, it is necessary to use a binder having a minimum film-forming temperature (MFT) of at most 10xc2x0 C., preferably at most 5xc2x0 C.
By satisfying these conditions, it is possible to form a coating film having excellent freezing-thawing stability, low temperature film-formability, anti-blocking property and water resistance without using or with only a very small amount use of a VOC as an anti-freezing agent or a coalescing agent.
Also, in the present invention, a Tg value of an emulsion polymer can be calculated in accordance with the following FOX formula.
1/Tg=W1/Tg1+W2/Tg2+ . . . +Wi/Tgi+ . . . +Wn/Tgn 
(In the above FOX formula, Tgi(K) represents a glass transition temperature of a homopolymer of each monomer constituting a polymer comprising n kinds of monomers, and Wi represents a mass % of each monomer, i.e. (W1+W2+ . . . +Wi+ . . . Wn=1.)
In the present invention, when a Tg value of an emulsion polymer of the outermost phase of different phase structure emulsion particles is less than xe2x88x9250xc2x0 C., a coating film obtained therefrom becomes poor in pollution resistance and water resistance. On the contrary, when the Tg value exceeds 10xc2x0 C., film-formability becomes unpreferably poor at a low temperature.
On the other hand, when Tg values of emulsion polymers of all phases constituting inner phases are less than 30xc2x0 C., a coating film obtained therefrom becomes poor in anti-blocking property and other physical strengths, and on the contrary, when the Tg values exceed 110xc2x0 C., it is not preferable since the reaction does not satisfactorily proceed.
Also, when a MFT value of a binder exceeds 10xc2x0 C., film-formability becomes unpreferably poor at a low temperature in winter.
Hereinafter, an ethylenic unsaturated monomer used for forming different phase structure emulsion particles is explained.
As described above, it is necessary for an ethylenic unsaturated monomer used for an emulsion polymer forming the outermost phase to contain an ethylenic unsaturated monomer having at least one of a polyethylene glycol chain and a polypropylene glycol chain as the essential component.
Said monomer is represented by the following formula (1), (2) or (3).
Formula (1):
CH2xe2x95x90C(R1)xe2x80x94C(xe2x95x90O)xe2x80x94Oxe2x80x94[Xxe2x80x94O]nxe2x80x94R2 
(Wherein R1 is H or CH3, R2 is H or a C1-C8 alkyl group, X is xe2x80x94(CH2)2xe2x80x94 or xe2x80x94CH2CH(CH3)xe2x80x94, and n is an integer of from 1 to 30.)
Formula (2):
CH2xe2x95x90C(R1)xe2x80x94(CH2)mxe2x80x94Oxe2x80x94[Xxe2x80x94O]nxe2x80x94R2 
(Wherein R1, R2, X and n are as defined in the above formula (1), and m is an integer of from 1 to 30.)
Formula (3):
CH2xe2x95x90CR1xe2x80x94C(xe2x95x90O)xe2x80x94Oxe2x80x94(CH2CH2O)mxe2x80x94[CH2xe2x80x94CH(CH3)xe2x80x94O]nxe2x80x94R2 
(Wherein R1 and R2 are as defined in the above formula (1), and m and n are an integer of from 1 to 30.)
If the amount of a monomer represented by the above formulas in the total ethylenic unsaturated monomers forming the outermost phase is less than 1 mass %, freezing-thawing stability of a paint becomes poor, and on the contrary, if this amount exceeds 20 mass %, a coating film obtained therefrom becomes unpreferably poor in water resistance.
These monomers can be easily prepared, for example, by addition-polymerizing ethylene oxide and/or propylene oxide with (meth)acrylic acid, allyl alcohol or the like and then optionally by ether-forming with a C1-C8 alkyl group. Examples of these monomers include monomers of tradenames such as xe2x80x9cMA-30xe2x80x9d, xe2x80x9cMA-50xe2x80x9d, xe2x80x9cMA-100xe2x80x9d, xe2x80x9cMA-150xe2x80x9d and xe2x80x9cMPG-130MAxe2x80x9d (manufactured by Nippon Nyukazai K. K.), xe2x80x9cBlenmer PExe2x80x9d, xe2x80x9cBlenmer PPxe2x80x9d, xe2x80x9cBlenmer AP-400xe2x80x9d, xe2x80x9cBlenmer AE-350xe2x80x9d, and xe2x80x9cBlenmer PEPxe2x80x9d (manufactured by Nippon Yushi K. K.), and the like.
Also, various ethylenic unsaturated monomers used conventionally in the preparation of acrylic resin can be used without any restriction as comonomers copolymerizable with monomers represented by the above formulas.
Examples of these comonomers include (meth)acrylate type monomers such as methyl(meth)acrylate, ethyl(meth)acrylate, n-propyl(meth)acrylate, isopropyl(meth)acrylate, n-butyl(meth)acrylate, isobutyl(meth)acrylate, pentyl(meth)acrylate, 2-ethylhexyl(meth)acrylate, decyl(meth)acrylate, dodecyl(meth)acrylate, stearyl(meth)acrylate, xcex1-chloroethyl(meth)acrylate, cyclohexyl(meth)acrylate, phenyl(meth)acrylate, methoxyethyl(meth)acrylate, ethoxyethyl(meth)acrylate, methoxypropyl(meth)acrylate and ethoxypropyl(meth)acrylate; styrene type monomers such as styrene, methylstyrene, chlorostyrene and methoxystyrene; carboxyl group-containing monomers such as (meth)acrylic acid, crotonic acid, itaconic acid, itaconic acid half ester, maleic acid and maleic acid half ester; hydroxyl group-containing monomers such as 2-hydroxyethyl(meth)acrylate, 2(3)-hydroxypropyl(meth)acrylate, 4-hydroxybutyl acrylate and mono(meth)acrylic acid ester of allyl alcohol polyhydric alcohol; amide group-containing monomers such as (meth)acrylamide and maleinamide; amide group-containing monomers such as 2-aminoethyl(meth)acrylate, dimethylaminoethyl(meth)acrylate, 3-aminopropyl(meth)acrylate, 2-butylaminoethyl(meth)acrylate and vinylpyridine; epoxy group-containing monomers and oligomers obtained by reacting an active hydrogen atom-containing monomer with an epoxy compound having at least 2 glycidyl groups, glycidyl(meth)acrylate or allylglycidyl ether; N-methylol acrylamide having a N-methylol group, vinyl acetate, vinyl chloride, ethylene, butadiene, acrylonitrile, dialkyl fumarate, and the like.
As described above, these comonomers are used optionally in such a combination as to provide an emulsion polymer forming the outermost phase having a Tg value of from xe2x88x9250xc2x0 C. to 10xc2x0 C. and a binder having a MFT value of at most 10xc2x0 C.
Also, as an ethylenic unsaturated monomer used for an emulsion polymer forming at least one phase of inner phases of different phase structure emulsion particles, the same ethylenic unsaturated monomers as illustrated above can be used, but it is necessary to use them optionally in such a combination as to provide an emulsion polymer having a Tg value of from 30 to 110xc2x0 C. and a binder having a MFT value of at most 10xc2x0 C.
The emulsion polymer for forming the outermost phase and the emulsion polymer for forming at least one phase of the inner phases are prepared by using monomers in such a combination as to satisfy the above required Tg and MFT values, but it is appropriate to select monomers to be used, in such a manner as to make a Tg value difference between the both emulsion polymers at least 30xc2x0 C. and to provide a binder comprising different phase structure emulsion particles, the MFT value of which is lower than a MFT value of a polymer obtained by uniformly emulsion-polymerizing total monomers for the both emulsion polymers at one stage.
Also, when using a carboxyl group-containing monomer as an ethylenic unsaturated monomer, it may be neutralized with a neutralizing agent such as ammonia, dimethylethanolamine or triethanolamine.
A resin composition for an aqueous paint of the present invention contains different phase structure emulsion particles obtained by the above-mentioned multi-stage emulsion polymerization method as a binder, and this binder may be used in a dispersion (emulsion) state in water as a clear paint, but in order to impart various functions to a paint, it is preferable to incorporate various additives such as a dispersant, an anti-settling agent, a thickening agent, an antiseptic agent, a UV ray-absorber, or a photostabilizer.
A paint obtained in this manner is applicable to various inorganic materials, a metal material, a wood material or a plastic material, and provides an excellent coating film by air-drying or force-drying at a temperature of at most 100xc2x0 C.